There is a growing demand in the mobile communication area for a system having the ability to download large blocks of data to a Mobile Station (MS) on demand at a reasonable rate. Such data could for example be web pages from the Internet, possibly including video clips or similar. Typically a particular MS will only require such data intermittently, so fixed bandwidth dedicated links are not appropriate. To meet this requirement in UMTS, a High-Speed Downlink Packet Access (HSDPA) scheme is being developed which may facilitate transfer of packet data to a mobile station at up to 4 Mbps.
There are a number of parameters which a Base Station (BS) needs to set for the first transmission of a packet, as well as for subsequent transmissions and retransmissions. Such parameters may include the transmission power, Modulation and Coding Scheme (MCS), spreading factor and number of channelisation codes (in a spread-spectrum system such as UMTS), and delay between retransmissions of a particular packet (or the transmission priority assigned for retransmissions).
In known radio communication systems, at any one time a MS generally communicates with a single Base Station. During the course of a call the MS may wish to investigate transferring to another BS, for example when the quality of the communication link deteriorates as the MS moves away from its BS, or when the relative traffic loading of different cells requires adjusting. The process of transferring from one BS to another is known as handover.
In a system operating according to the current UMTS specifications, the MS maintains a list of BSs known as the “active set” with which it is expected that radio links of reasonable quality can be maintained. When the MS is in dedicated channel mode, and there are multiple BSs in the active set, the MS is in “soft handover” with the BSs in the active set. In this mode uplink transmissions are received by all BSs in the active set, and all BSs in the active set transmit substantially the same downlink information to the MS (typically the data and most of the control information would be the same, but power control commands could be different). A drawback of this “soft handover” approach is that the uplink and downlink transmission powers cannot be optimised for each individual radio link, as only one set of power control commands is transmitted in the uplink, while the power control commands transmitted over the downlink from different BSs may result in conflicting requirements for the uplink transmission power.
The normal soft handover procedure is particularly suitable for real time services such as voice links, where a continuous connection must be maintained. For packet data links, however, it can be advantageous to select the optimum BS for the transmission of each data packet to a MS, to allow for dynamically changing radio link and traffic conditions. Improved system throughput can be achieved if the selection of the optimum BS is made immediately prior to transmission of each packet, minimising the number of packets received in a corrupted state and also minimising total transmitted power per packet.
A problem with the process of fast site selection, as outlined above, is that it may interact badly with the design of an ARQ (Automatic Repeat reQuest) process. An example of such an interaction arises if the selected BS is changed before a data packet has been correctly received. A further problem with a packet data system is the need for signalling a plurality of separate sets of parameters relating to packet transmission.